The Moon River crooner lost his battle with cancer at the age of 84, and stars including Osmond's siblings Marie and Donny paid tribute to the star.
Alan has now penned a moving piece in honour of Williams, who took the singing family under his wing at the beginning of their careers.
In a post on TheFamily.com, he writes, "When I heard this morning that Andy Williams had died, my heart felt a major loss; yet there was a peaceful feeling of love that warmed my soul for a dear friend. I knew he had been struggling with a health challenge but was told a few weeks ago that he was planning on returning to the stage.
"We have known Andy Williams for a long time and we loved him. He was like an older brother and a part of our Osmond Family. (On) behalf of my family, we offer our everlasting love to Andy and his family. I am sure that my parents who have also gone on to the other side were there to welcome Andy along with Andy's parents. That's how close we were.
"You can't find a better singer than Andy Williams! Andy was the Best! You can't find a more loyal friend and supporter than Andy Williams. We were Best of friends and families. We Osmonds have been Blessed because of Andy Williams and we only wish him and his family and loved ones the VERY Best OF LIFE FOR ALL ETERNITY!"
Jackie Jackson has also paid tribute to the late star, writing on his Twitter.com page, "Rest in peace Andy Williams - you were one of my heroes - a great inspiration watching you perform. We studied Andy Williams way of singing as children, especially 'Moon River'.
Steve Martin added, "Andy Williams was a beautiful singer, a fine, gentle, man, and had a love and keen eye for art. Also a friend. He had an enviable life", while actress Patty Duke offered, "RIP Andy Williams. I will forever remember working with you and your music. My thoughts and prayers with ur family."